Sound pick-up for audioconferencing or videoconferencing equipments is difficult to achieve in the so-called “handsfree” context. Handsfree operation raises a dynamic range problem for the signals processed. Typically, the level on the microphone varies over a very wide range. This strong level variation is due in particular to the remoteness or closeness of the speaker from or to the microphone, and to the natural variety of speech power between individuals.
The most acute cases are those in which the people using the equipment may be situated either very close to or very remote from the microphone or microphones. In the very close case, this situation can cause saturations. In the very remote case, it can result in poor sound pick-up for speech.
To correct the problem of the low levels, there are commercially available electronic appliances, or even voice-over-IP software versions, offering automatic control of the sound pick-up level. These devices can be applied to telecommunication equipments. Many earlier systems use gain variation, in particular an automatic attenuation of the signal sent over the telecommunication network. For example, document FR-2 841 721 proposes to use the presence of a signal on one or several loudspeakers to calculate the gain correction.
There are also solutions in integrated circuit form for amplifying the audio signal. However, these circuits do not take account of a reference signal such as the signal feeding the loudspeaker or speakers.
The “Skype” voice-over-IP software uses an automatic gain control technique on the microphone signal. This technique is not applied in the so-called “multichannel” context (several microphones and/or several loudspeakers) and does not take account of a reference signal.